1. Field of the Invention
The present invention relates to a process for carbonylating alcohols by means of a rhodium or iridium based catalyst in the presence of a halide used as a promoter, carried out in an ionic liquid medium with efficient catalyst recycling. The homogeneous system obtained at the end of the reaction is distilled. The distillation residue is constituted by the catalyst dissolved in a non-aqueous ionic liquid. Said ionic liquid comprises at least one quaternary ammonium and/or phosphonium cation Q+ and at least one anion A−. The catalyst comprises at least one complex of rhodium or iridium and the promoter is constituted by at least one halide.
Carbonylation of alcohols, in particular methanol, is a reaction of major industrial importance in the manufacture of acetic acid used in a wide variety of applications. That product is directly involved in the manufacture of vinyl acetate and acetic anhydride and can be used as a reaction solvent in the production of terephthalic acid.
2. Description of the Prior Art
Many references deal with the production of acetic acid by methanol carbonylation. Examples that can be cited are the articles by BASF at the end of the 1960s (Hydrocarbon Processing, November 1966, vol. 45, no 11, p 141 and Hydrocarbon Processing, November 1973, p 92) which describe the methanol carbonylation reaction under very severe conditions (60 MPa, 230° C.) in the presence of a complex based on cobalt promoted by an iodine derivative. In the 1970s, Monsanto commercialized an improved process (Monsanto (1973), U.S. Pat. No. 3,769,329) which functions under much milder temperature and pressure conditions (3 to 4 MPa, 180–220° C.) in the presence of a rhodium based complex promoted by methyl iodide. That process is distinguished by a very high yield of acetic acid (99%) linked to the presence of a high concentration of water in the reaction mixture. In 1980, Hoechst-Celanese improved the Monsanto process by adding a promoter based on lithium iodide or sodium iodide (Hoechst Celanese, Corp (1991), U.S. Pat. No. 5,001,259), which considerably reduced the concentration of water necessary in the Monsanto process and thus limited problems with the formation of by-products by reducing the “water gas shift” side reaction. Finally, in 1996, BP Chemicals sold a process based on iridium (Cativa™) comparable with the Monsanto process with a low water concentration (Chem Br 32 (1996) 7 and Chem Ind (London) 483 (1996)). That process was a considerable improvement over the earlier processes, for example in increasing the reaction rates, reducing liquid by-products and increasing the yield with respect to carbon monoxide.
The majority of said processes use homogeneous catalysts dissolved in an organic phase constituted by reagents and products, which sometimes renders separation complex and expensive.